Helmets.

After Richmond's accident in Afghanistan Rich and I were discussing helmets with some other people on Facebook. All climbing helmets meet the same minimum standard but Rich in particular was curious whether a large, sharp object would have penetrated our expanded plastic foam (EPF) Petzl Meteor helmets any more than Richmond's 'hardshell over foam' BD Half Dome.

So I started looking around at the pros and cons and this is what I found.
Dr. Mark Taylor of the University of Leeds is the Climbing Helmet guru- his PhD focused on the impact absorbing mechanisms of helmets. Paul from Peak Mountaineering has a good summary of a helmet testing workshop he presented for the BMC here.
Dan Middleton of the BMC Technical Committee has also written some very accessible articles on both the BMC website (here and here) and UKC. His summary from the latter is worth repeating:

"Shell/Cradle- e.g. Petzl Ecrin Roc

An internal webbing cradle holds the hard shell away from the head. During impact, the force on the head is reduced by stretching of the cradle, and also by progressive damage to the shell. Protection becomes minimal towards the rim of the helmet. Top impact and penetration resistance are often bolstered by the reinforcement of the top of the shell. Most designs are quite rugged and durable. 

Best for: Alpine, winter mountaineering, group use."

"Expanded Plastic Foam- e.g. BD Tracer, Petzl Meteor series

Made from expanded closed cell foam, with a thin cosmetic cover. Impact force is reduced by the gradual disintegration of the foam cells. These designs tend to only just pass the top impact and penetration tests. They do however give protection all the way to the rim of the helmet – the only type to do this. Damage from impact or lack of care can render them useless, so not ideal for long, serious routes.

Best for: Sport & outcrop climbing, ski-mountaineering."

"Shell/Foam e.g. BD Half Dome, Petzl Elios

The majority of modern designs fall into this category. They have a chinstrap but no cradle. A hard shell provides penetration resistance and durability, whilst a foam liner absorbs impacts. The thicker and more extensive this foam liner, the better the protection will be. Any soft open cell foam around the rim is purely cosmetic.

Best for: All-round climbing and mountaineering use."

"Finally, remember that all helmets only offer limited protection, however well designed they may be. Wearing one may stack the odds in your favour if the worst happens, but it won't remove the risk of injury. Individual judgement is still essential when deciding what is and isn't an acceptable level of risk."

(UKC 'Helmets- Everything You Need To Know' Dan Middleton Jun 2008)

Other points of note are that in the (admittedly unlikely) event of an off centre impact the EPF helmets tend to offer more uniform protection over all of the head than other models. They are however easy to damage in transit and not durable. 

Part of the testing to ensure helmets meet the standard includes resistance to penetration (which is why vents are on the side, not the top). I'm still a fan of my Petzl Meteor III and I'm glad the vents aren't as large as some models as I value the all over protection t gives me.

Choose the helmet you want based on an understanding of pros and cons. But try to avoid situations where you have to test them in the first place!